1. Field of the Invention
Firstly, in order to better understand the invention and the description below, it is worth giving some short information about the manufacturing of printed circuits to which the present invention is preferably applied.
2. Background
As it is known, printed circuits for electronic applications such as those used for computers (e.g. Personal computers), telecommunication apparatuses (telephones, radios, etc.) household appliances and so on, are obtained by overlapping several conducting layers incorporating the traces designed according to the layout of the circuit, which are separated by layers made of electrically insulating material.
Such configuration leads to a multilayer stack that allows the final printed circuits to be obtained by means of subsequent processing and finishing steps; the circuits can be of the rigid type, such as for example electronic cards (so called PCBs or printed circuit boards), or flexible type such as circuits used for small apparatuses or associated to moving parts of machines.
The several overlapped layers of the stack are closely joined together by means of an adhesive applied on an electrically insulating substrate; generally the latter is of textile or fibre type, made of glass, synthetic fibre or the like and it is impregnated with the adhesive such that it is usually called a pre-preg (the abbreviation for pre-impregnated).
Layers are usually joined and glued into suitable presses where the semifinished multilayer stack is subjected to a heating and compression cycle.
In order to obtain the final desired product, that is a printed circuit having the designed functional and structural properties, it is necessary to accurately overlap the conducting and insulating layers in the multilayer stack.
Therefore, to this end, the relative movements between a layer and the other one have to be prevented during the several steps for processing of the multilayer stack.
To this end, it is known to provide a spot welding of the stacked layers, such to keep them firm and therefore such to allow the multilayer stack to be handled: the present invention relates to such background.
The present invention relates to a new method and to a relevant apparatus for bonding at predetermined locations the layers of electronic circuits overlapped as a stack.
During the last decades several bonding techniques have been developed with more or less satisfactory results, based on the localized heating of the multilayer stack; for example apparatuses have been manufactured wherein the necessary thermal power is supplied by electrodes heated by heating elements or wherein radiant energy is used in the form of microwaves or electromagnetic induction.
For example, a method and an apparatus for performing it are known from the American patent U.S. Pat. No. 7,009,157 B2 to Lazaro Gallego, wherein the multilayer stack of printed circuits is bonded at predetermined locations.
The latter are arranged at the corners of the several overlapped layers and short-circuited turns are placed at the region thereof: induced currents are generated in the turns by applying a magnetic induction field thereto, which produce the heat necessary to locally melt the thermosetting resin that impregnates the insulating substrates (the pre-preg).
The locally hardened resin guarantees the layers to be joined at the locations where short-circuited turns are provided, such to achieve the desired solid configuration of the multilayer stack.
In order to apply the magnetic induction to the turns, the patent U.S. Pat. No. 7,009,157 provides an apparatus with induction heads which comprise a substantially “C” shaped core upon which a coil is wound, and two telescopic pole pieces: the multilayer stack is interposed between the pole pieces whose mutual distance can be adjusted such that they slightly press against the multilayer stack, so that a good contact and a better application of the magnetic induction field is guaranteed.
Although this type of heating is able to reach efficaciously even the inner areas of the multilayer stack, it has, however, some considerable drawbacks.
Firstly, it allows the multilayer stack to be bonded only at individual restricted locations, namely those where short-circuited turns are placed.
Consequently, the bonding process is not so much flexible since if the shape of the multilayer stack and/or the position of the short-circuited turns change, it is necessary to adjust the position of the induction heads.
Therefore, the magnetic configuration thereof changes as the thickness of the multilayer stack changes, since it is necessary to translate the pole pieces in order to adjust their mutual distance: therefore the flux intensity consequently changes (all the other conditions being equal) when multilayer stacks with different thicknesses have to be processed.
Similar remarks are also valid in relation to another induction bonding apparatus known in the patent application US 2010/0212945 A1 to Faraci, which describes a bonding machine wherein induction heads are composed of “E” shaped cores separated from each other.
This shape of the magnetic cores involves a flux passing through the central leg upon which the excitation coil is wound, different than the side legs that convey a respective flux that is about one half of that of the central leg.
The fact that it is necessary to bond the multilayer stacks at the shorted-circuit turns, affects the flux linked to the turns that essentially is given by the vector sum of that flowing through the central leg and those associated to the side legs of the core.
Accordingly the flux effectively linked to the turn is lower than that generated by the induction coil on the central leg, with a waste of energy greater due to an unbalanced distribution of the fluxes.
This type of solution may further results in a non optimal magnetic flux for quickly bonding a thick multilayer stack, since the magnetic flux is not homogeneously distributed among the layers of the multilayer stack causing them not to be uniformly bonded, due to the different temperatures reached in the several layers during the bonding phase.
Therefore the technical problem of the present invention is to overcome the above mentioned drawbacks of the known bonding systems.